<form action="{:url($uploadUrl)}" method="post" enctype="multipart/form-data">
    <input type="hidden" name="id" value="{$task.id}" />
    <ul>
        {volist name="task.files" id="file"}
        <li><a href="{$file.url}">{$file.create_time}</a></li>
        {/volist}
        <li><input type="file" name="file" class="form-control"></li>
        <li><button class="btn btn-default" type="submit">修改任务</button></li>
    </ul>
</form>